Current transformers
A current transformer converts a high primary current into a small, standardised secondary current – usually 5 A or 1 A. Only then can meters, measuring instruments and protection relays safely record the current of an installation. The right design depends on whether the conductor can be opened for mounting, what the measured value is used for and which accuracy is required.
At Stromwandler24 you will find slip-on and wound-primary CTs (ALO, WALO), calibrated CTs for billing (ALO-E, WALO-E), split-core CTs for cables and busbars, and protection CTs. The CT finder leads you to a suitable variant in a few steps, the VA calculator checks the burden, and the ratio pages such as 400/5 A list every type for a given ratio. If a question remains, write to us using the contact form.
Not sure which transformer fits?
The assistant asks step by step about purpose, mounting and current and shows the matching articles.
Sub-groups
Products
Designs in the shop at a glance
| Design | When | Primary current | Secondary | Classes |
|---|---|---|---|---|
| Plug-on CTs | conductor is free: new installation, distribution board, cabinet | up to 6000 A | 1 A, 5 A, 333 mV | 0.5 to 3 |
| Wound primary CTs | small primary currents, conductor on the terminals | 1 A – 40 A | 1 A, 5 A, 333 mV | 0.5, 1 |
| Split-core CTs | conductor stays connected: retrofit in operation | 60 A – 3000 A | 1 A, 5 A, 333 mV | 0.5, 1, 3 |
| Calibrated CTs | billing with an MID meter | 1 A – 5000 A | 1 A, 5 A | 0.2S to 0.5 |
| Protection CTs | protection relays and circuit breakers | 75 A – 4000 A | 1 A, 5 A | 5P5 to 10P10 |
| Summation CTs | several feeders on one meter | 2 – 9 inputs | 1 A, 5 A | 0.5, 1 |
Every ratio has its own page with all matching versions, for example current transformer 400/5 A; all ratios and designs are listed below.
What matters when buying a current transformer
A current transformer converts the current in your cable or busbar into a small measuring current of 5 A or 1 A that meters, instruments and protection relays work with. Four values decide the right version: the ratio (primary and secondary current), the design to suit the installation, the accuracy class and the rated output in VA. All four are in the designation and datasheet of every variant in the shop.
With these values at hand, the transformer finder or the filters of this list take you straight to the orderable version with its delivery date. If you are unsure, the guide choosing a current transformer gives the order of decisions.
Choosing the ratio: primary and secondary current
The ratio is printed on every transformer, for example 250/5 A: at 250 A in the conductor, 5 A flow to the meter. Choose the primary current just above the highest operating current of the conductor; a clearly oversized transformer is less accurate at low load because the permissible errors to IEC 61869-2 grow below 20 % of rated current. The instrument dictates the secondary current: 5 A is common, 1 A pays off on long measuring leads because cable losses rise with the square of the current.
Every ratio has its own page in the shop, such as 400/5 A or 100/5 A; all versions with 1 A secondary are collected on current transformers 1 A. The CT factor for your meter is worked out in calculating a current transformer.
Accuracy class: 0.5, 0.5S, 1 or 3?
The class states the permissible error at rated current: class 1 allows ±1 %, class 0.5 allows ±0.5 %, each between 100 % and 120 % of rated primary current. At 20 % and 5 % load the limits widen. Classes 0.2S and 0.5S are specified down to 1 % of rated current and keep the full class value from 20 %; they are the choice when the installation often runs at low load. Class 3 is only specified between 50 % and 120 % and is enough for display only.
Billing third parties requires calibrated transformers together with an MID meter; the class the metering point operator demands is in its connection conditions. For energy monitoring without billing class 1 is usually sufficient. Error limits per operating point are in the class comparison, the background in the guide accuracy classes.
Rated output in VA: the burden must fit
Meter, instrument and measuring lead load the transformer; this burden in VA must be covered by its rated output. If it is higher, the transformer reads low; if it is much lower than the rated output, the class no longer applies either, because it is only guaranteed between 25 % and 100 % of rated burden. Example: 10 m of measuring lead in 2.5 mm² copper consume 3.56 VA at 5 A secondary, only 0.14 VA at 1 A; add the meter consumption from the datasheet.
The VA calculator works with the length and cross-section of your lead; formula and table are in the guide calculating the burden.
Connection: P1/P2, S1/S2 (k/l), earthing and short-circuiting
P1 (formerly K) faces the supply, P2 (L) the load; the secondary terminals S1 and S2 (formerly k and l) lead to the current path of the meter on the same phase. If the secondary circuit is earthed, it is earthed at exactly one point – at S1 or S2, the same on all transformers of a metering point; which terminal is specified by the grid operator’s connection conditions or the design.
Under load the secondary circuit must never be open, otherwise dangerous voltages arise at the terminals. Before a device is disconnected the transformer is short-circuited – with a CT test terminal or, on the ALO plug-on CTs, with the short-circuit bridge in the rotary cover. Wiring diagram and sequence: connecting a current transformer.
Ordering with a delivery date
Every variant shows a fixed delivery date; stock items are marked “from stock”. Datasheet and dimension drawing come with every version. Ratios not in the list can be requested via the contact form.
Frequently asked questions
Which current transformer do I need?
Primary current just above the highest operating current of the conductor, secondary current to suit the instrument (5 A or 1 A), class by purpose (billing: calibrated, 0.5 or better; monitoring: usually 1) and rated output to suit instrument and lead. The design follows from the installation: plug-on CT with a free conductor, split-core CT for retrofits.
What does 400/5 A mean?
At 400 A in the conductor the transformer delivers 5 A to the meter; the CT factor is 80. At 200 A it delivers 2.5 A and the meter shows 200 A again after multiplying by 80.
Class 0.5 or 0.5S?
If the load is usually between 20 % and 120 % of rated current, class 0.5 is sufficient. If the installation often runs far below that, 0.5S is more accurate: it is specified down to 1 % of rated current and allows only ±0.75 % at 5 % instead of ±1.5 %.
Do I have to shut down the installation to install it?
For plug-on CTs yes, because the conductor passes through the window. Split-core CTs are opened and placed around cable or busbar; the primary circuit stays in operation, the rules for work on live parts still apply.
Why is a current transformer short-circuited?
Because it keeps driving its secondary current while primary current flows: dangerously high voltages arise at open terminals. S1 and S2 are therefore short-circuited before every disconnection.
Which transformers may I use for billing?
Calibrated (conformity-assessed) transformers together with an MID meter – in the shop the ALO-E and WALO-E series with PTB type approval in classes 0.2S, 0.2, 0.5S and 0.5.
1 A or 5 A secondary?
5 A for meters close to the transformer. For long measuring leads 1 A: the cable loss is then one twenty-fifth of the value at 5 A. All 1 A versions are on the page current transformers 1 A.
Is there a current transformer with a 4–20 mA output?
No, a current transformer delivers 1 A, 5 A or 333 mV. The 4–20 mA signal is produced by a downstream transducer with a 1 A or 5 A current input.
Current transformers by ratio
Every ratio has its own overview with all matching types; each design has its own pages. Also: current transformers 1 A · current transformers for meters · overview by ratio and design